Are there any advantages to keeping bitcoin in Coinbase?
No.
Or is it more advantageous to move into a private wallet controlled only by myself (such as Samouri Wallet)?
You should buy a hardware wallet and store your coins there.
If you let money in Coinbase, your coins aren't really yours. There are 3 main risks imo:
1 - Your account may be frozen due to some crazy new regulation/anti-money laundering policy/etc.
2 - Coinbase may be hacked and you lose your coins
3 - Coinbase may just disappear and run away with your money. This is less likely, but why take any risk?
With about 80 usd you can buy a ledger nano and you will have a 99.99% secure wallet. You won't get hacked, it is very easy to use and your coins will be yours forever.
Your coins will be really yours. There is no way to confiscate your coins from you. Even the government, or a judicial sentence, nothing can take your coins away from you.
The proper way to use bitcoins is buy storing it in your wallet. You will be responsible for keeping your private keys safe, noted down in a piece of paper (which is all you need to spend them)